Whoa! That's a big one...and very nice to boot. As a bridge troll, I think that would definitely make a nice build.
What I'd recommend is that you build the "vertical" parts of the bents from small steel angle iron attached to square wooden bases (hidden under dirt and deck), then detail the cross members with Plastruct products. The deck itself can be wood, painted steel gray and detailed with more Plastruct. We did something similar with our curved modern trestle in the background. The deck is wood with resin girders on the side. The bents are steel 1/4" angle iron fastened to wood blocks top and bottom (angled ever so slightly) and completely trimmed out in Plastruct. The steel supports the weight but the bents look nice and light.
